Monotertiarybutyl hydroquinone is well known and is prepared by different processes. U.S. Pat. No. 2,722,556 the disclosure of which is incorporated here by reference in its entirety, discloses a process for preparing monotertiarybutyl hydroquinone by reacting hydroquinone with isobutylene or tertiarybutyl alcohol. Pure or food-grade tertiarybutyl hydroquinone is obtained by purifying impure monotertiarybutyl hydroquinone. Food-grade tertiarybutyl hydroquinone is very desirable and is useful for preventing oxidation in food products, thus preserving freshness of flavor and aroma. The required specifications for food-grade tertiarybutyl hydroquinone are: Assay 99% Min., Arsenic 3 ppm Max., t-Butyl-p-benzoquinone 0.2% Max., Heavy Metals 10 ppm Max., Hydroquinone 0.1% Max., Toluene 0.0025% Max., and passing the Ultraviolet Absorbance Test.
The main impurity in monotertiarybutyl hydroquinone (MTBHQ) is the by-product of the alkylation of hydroquinone with isobutene, namely 2,5 ditertiarybutyl hydroquinone (DTBHQ). The current practice for the preparation of food-grade tertiarybutyl hydroquinone (TBHQ) is by recrystallization. This recrystallization entails completely dissolving crude water wet MTBHQ crystals in toluene, decanting the water and filtering the hot solution to a crystallizer. The MTBHQ is crystallized, and the majority of the DTBHQ remains in the toluene mother liquor. The toluene is removed from the wet crystals by further processing steps. The product is then isolated, dried and then pulverized prior to packaging for sale. This process is expensive and is very intensive in both labor and equipment requiring long batch cycle times and specialized equipment. Consequently, the cost to purify MTBHQ to food-grade TBHQ by this process is high.
Another possible purification of MTBHQ to food-grade TBHQ is distillation at high vacuum, however, the relative volatility of MTBHQ and DTBHQ is close to one so that the separation would require many stages of rectification and high reflux ratios. Operating under high vacuum at high temperature allows oxygen to enter the distillation column, causing some oxidation of the product. In addition, unless all traces of acid are neutralized, the temperatures required for distillation causes MTBHQ to dealkylate, producing hydroquinone which contaminates the product.
In light of the above, it would be very desirable to be able to produce food-grade TBHQ in a less complicated, less expensive process avoiding the above problems.